Different Types of Algae in Aquarium

If you're staring at a smudge, a fuzz, or a slimy sheet in your tank and wondering what it is, you've come to the right place. Almost every algae problem in a freshwater aquarium is one of nine recognizable types, and each one tells you something specific about light, nutrients, or how mature the tank is. This guide is a practical identification chart for the different types of algae in aquarium setups, with what triggers each and where to focus your fix.

For the full balancing strategy that prevents all of these, see the aquarium algae control guide. This page is the field guide that goes with it.

Green spot algae

Hard, bright-green dots, usually 1-3 mm wide, stuck firmly to glass and the oldest, slowest leaves like Anubias. You can't wipe them off with a finger; they need a scraper. GSA is the textbook sign of low phosphate combined with strong, long lighting. Counterintuitively, many planted-tank keepers fix it by dosing more phosphate, not less, while shortening the photoperiod. Nerite snails are one of the few animals that genuinely eat it.

Green dust algae

A fine, even green film that coats the glass and looks like a lawn under good light. Wipe it and it returns within a day or two. Green dust is a free-floating algae stage; the textbook approach is to leave it untouched for two to three weeks while it completes its life cycle, then do a deep wipe and large water change to remove the spore stage in one shot. Cutting the photoperiod helps prevent it returning.

Green water (algae bloom)

The whole tank turns pea soup. This is a free-floating microalgae bloom triggered by excess light plus excess nutrients (often after disturbing substrate, overfeeding, or a missed water change). It's harmless to most fish and shrimp short term, but a heavy bloom can crash oxygen overnight. The fastest cure is a UV sterilizer or a blackout: total darkness for three to four days plus heavy aeration usually clears it. After that, fix the photoperiod and feeding routine that caused it.

Green hair and thread algae

Long, bright-green strands draped over plants, hardscape, and filter intakes. "Hair" is finer; "thread" is thicker and stringier, but the cause is the same: too much light or a nutrient imbalance, often excess iron and high light without matching CO2. Manual removal works well — twirl it out with a toothbrush — and Amano shrimp graze the soft new growth aggressively. Long term, shorten the photoperiod and add fast-growing stem plants to outcompete it.

Black beard algae (BBA)

Dark green to black tufts of stiff, bushy fuzz that lock onto leaf edges, driftwood, and equipment. BBA almost always signals unstable CO2 or fluctuating flow in a planted tank — areas right next to the filter outflow are usually the worst. Cleanup animals rarely touch it; Siamese algae eaters are the main exception. The reliable fix is to stabilize CO2, improve flow distribution, prune the worst leaves, and spot-treat persistent patches with hydrogen peroxide or liquid carbon outside the tank.

Brown diatoms

A dusty brown coating on glass, substrate, and plants in a young tank. Wipes off easily, often with one finger. Diatoms feed on silicates and thrive while a new tank's biofilter and microflora are still establishing. The good news is they're a phase, not a chronic problem — most diatom outbreaks fade on their own within a few weeks. Otocinclus catfish and nerite snails happily clear what's left.

Blue-green algae (cyanobacteria)

Slimy, sheet-like blue-green to dark green coating that lifts off in slabs and often has a strong earthy or musty smell. Despite the name, this isn't algae at all — it's cyanobacteria, a photosynthetic bacterium. That's why it shrugs off the usual algae-eaters and ordinary light cuts. It thrives where flow is poor, organic waste accumulates, and nitrate is bottoming out (yes, too low nitrate often triggers it).

It's also the one type that can release toxins as it dies, so it's worth taking seriously. The standard approach is a combination of physical removal, improving flow into dead spots, restoring nitrate to a measurable level, and a 3-day blackout. Some keepers also use erythromycin-based treatments because cyanobacteria are bacteria; if you go that route, follow the product's labeled directions and remove any biological media as instructed — this is one to research carefully before dosing.

Staghorn algae

Stiff, branching gray-green to greenish strands that look like miniature deer antlers, usually growing on plant edges and equipment. Staghorn turns reddish in alcohol or peroxide, which is a quick test to distinguish it from BBA's denser tufts. Like BBA, it's tied to poor CO2 stability and high organic load. Manual removal plus better flow, more frequent water changes, and a shorter photoperiod are the proven path.

Cladophora

Coarse, branching dark-green strands that feel like wet wool and have a distinctive musty smell when crushed. It's the algae you most want to avoid because, once established, it attaches stubbornly to substrate, hardscape, and even plant stems and is famously hard to eradicate. Most cleanup animals ignore it. The standard approach is aggressive manual removal, spot-treating with liquid carbon, and a stricter light/nutrient regime — sometimes for months.

A quick identification cheat sheet

• Hard green dots on glass? Green spot algae — usually low phosphate + strong light.
• Even green lawn on glass? Green dust — let one cycle complete, then wipe + water change.
• Whole tank green? Green water bloom — UV or 3-day blackout.
• Long green strands? Hair or thread algae — excess light or nutrient imbalance.
• Black bushy tufts? Black beard algae — unstable CO2 or flow.
• Brown dust in a new tank? Diatoms — usually a phase that ends on its own.
• Slimy sheets that smell earthy? Cyanobacteria — bacterium, not algae; needs a different approach.
• Antler-like gray-green strands? Staghorn — CO2 and organics issue.
• Coarse, wool-like dark green strands? Cladophora — manual removal and patience.

A test kit makes this much easier. If you can rule in or out low phosphate, low nitrate, or runaway nitrate, you're already halfway to the right fix.

Lighting is usually the common thread

Across almost every algae type above, two things consistently shorten outbreaks: a sensible photoperiod (6-7 hours on a timer) and light intensity matched to the tank. Blasting a low-tech tank with a high-PAR light is the single most common reason hobbyists end up cycling through three or four of the algae types on this page in a single year. If you're due for a new fixture, see our planted tank light guide for matching intensity to setup type.

Where to go from here

Identifying the algae is half the battle — now you know which lever (light, photoperiod, CO2, nutrients, flow, or maturity) to pull. For the full step-by-step plan, the cleanup crew that earns its keep, and the prevention checklist, head back to the aquarium algae control guide. Get the balance right and most of the types on this page simply stop showing up.